Page:Popular Science Monthly Volume 56.djvu/71

of a remarkable phenomenon which has made Marconi's system of wireless telegraphy possible. In order that an electrical current may flow through a mass of particles of a metal, a mass, for instance, of iron filings, it is necessary either to compress them or to cause a minute spark or electrical discharge between the particles. Now, it is supposed that the magnetic whirls, in embracing the distant receiving circuit, cause these minute sparks, and thus enable the electric current from the battery B to work a telegraphic sounder or bell M. The metallic filings are inclosed in a glass tube

Fig. 3.—Photograph of the electric lines which emanate from the end of the wire at the sending station, and which are probably reproduced among the metallic filings of the coherer at the receiving station.

between wires which lead to the battery, and the arrangement is called a coherer. It can be made small and light. Fig. 6 is a representation in full size of one that has been found to be very sensitive. It consists of two silver wires with a few iron filings contained in a glass tube between the ends of the wires. It is necessary that this little tube should be constantly shaken up in order that after the electrical circuit is made the iron filings should return to their non-conducting condition, or should cease to cohere together, and should thus be ready to respond to the following signal. My colleague. Professor Sabine, has employed a very small electric motor to cause the glass tube to revolve, and thus to keep the filings in motion while signals are being received. Fig. 7 shows the arrangement of the receiving apparatus.

The coherer and the motor are shown between two batteries, one of which drives the motor while the other serves to work the